Variable Power Demand Research Articles

Voltage Stabilization: A Critical Step Toward High Photovoltaic Penetration

The increasing photovoltaic (PV) power sources connected to low-voltage (LV) distribution networks generate a new grid environment featuring various types of power generations near consumers and bidirectional active and reactive power flows. However, the large-scale deployment of PVs is hindered by the power quality problems, particularly voltage deviation. To overcome this obstacle, proper mitigation techniques should be developed to eliminate the negative impacts of high-PV penetration in LV networks. This article provides an in-depth review of recently developed technologies that prevent voltage deviation in LV grids with PVs. Following an investigation of the voltage fluctuation phenomena along the distribution feeder due to variable PV output and power demand, the mathematical relationship between power flow and voltage level is revealed. The solutions that mitigate the voltage variation are then investigated and classified. Their effectiveness, advantages, and disadvantages are illustrated. Finally, the current trend in grid integration of PVs and other distributed generators (DGs) under future grid framework is discussed. Voltage Deviation Challenge.

Dynamic Capabilities of Multi-MHz Inductive Power Transfer Systems Demonstrated With Batteryless Drones

This paper presents the design of a multi-MHz inductive power transfer (IPT) system showcasing lightweight and energy-efficient solutions for non-radiative wireless power transfer. A proof of concept is developed by powering a drone without a battery that can hover freely in proximity to an IPT transmitter. The most challenging aspect, addressed here for the first time, is the complete system-level design to efficiently provide uninterrupted power flow while allowing for variable power demand and highly variable coupling factor. The proposed solution includes the design of lightweight air-core coils that can achieve sufficient coupling without degrading the aerodynamics of the drone, and the design of newly developed resonant power converters at both ends of the system. At the transmitting-end, a load-independent Class EF inverter, which can drive a transmitting-coil with constant current amplitude and achieves zero-voltage switching for the entire range of operation, was developed; and at the receiving-end, a hybrid Class E rectifier, which allows tuning for large changes in coupling and power demand, was used. For the demo, the range of motion of the drone was limited by a 7.5 cm nylon string tether, connected between the center of the transmitting-coil and the bottom of the drone. The design of the IPT system, including all the power conversion stages and the IPT link, is explained in detail. The results on performance and specific practical considerations required for the physical implementation are provided. An average end-to-end efficiency of 60% was achieved for a coupling range of 23%-5.8%. Relevant simulations concerning human exposure to electromagnetic fields are also included to assure that the demo is safe, according to the relevant guidelines. This paper is accompanied by a video featuring the proposed IPT system.

Effect of energy storage systems on automatic generation control of interconnected traditional and restructured energy systems

During major disturbances in electric power system (PS) penetrated with renewable energy sources, primary and supplementary automatic generation control (AGC) strategies usually show inefficiency in mitigating the frequency and power oscillations because of sluggish control action. The frequency and power deviations should be controlled to retain the generation-demand balance, which reinforce the quality and stability of overall PS. The fast-acting energy storage systems (ESSs) having very small time constants like capacitive energy storage (CES) and redox flow battery (RFB) are utilised in this study to improve these dynamic responses. To conduct the analysis, initially, a two-area nonreheat thermal PS with extra generations from wind turbine system (WTS) and dish-stirling solar thermal system (DSTS) is explored extensively, and then to validate the efficacy of the method, the approach is tested on two-area nonreheat thermal system having governor deadband (GDB) nonlinearity, reheat thermal, and restructured multisource thermal gas systems. An imperialist competition algorithm (ICA) optimised fuzzy PID-filter-(1 + PI) controller named as FPIDF-(1 + PI) is utilised as supplementary controller, and its performance with CES/CES-RFB is compared with ICA-optimised FPIDF with/without CES and existing optimal PI/PID/PIDF/FPID controller without CES. Investigation of dynamic responses for sudden variation in power demand unveils the superiority of the control approach compared with others regarding settling time, peak undershoot, and performance index. Analysing the impact of ESSs on the responses divulges that the amalgamation of CES-RFB in PS imparts better system dynamics. The robustness analysis suggests that ICA-optimised controller with ESSs performs excellently and robustly for ±25% variation in PS parameters, random load disturbances, and nonlinearities.

Handling uncertainties with affine arithmetic and probabilistic OPF for increased utilisation of overhead transmission lines

Large-scale integration of variable and unpredictable renewable-based generation systems poses significant challenges to the secure and reliable operation of transmission networks. Application of dynamic thermal rating (DTR) allows for a higher utilisation of transmission lines and effectively avoids high-cost upgrading and/or reinforcing of transmission system infrastructure. In order to efficiently handle ranges of uncertainties introduced by the variations of both wind energy sources and system loads, this paper introduces a novel optimization model, which combines affine arithmetic (AA) and probabilistic optimal power flow (P-OPF) for DTR-based analysis of transmission networks. The proposed method allows for the improved analysis of underlying uncertainties on the supply, transmission and demand sides, which are expressed in the form of probability distributions (e.g. for wind speeds, wind directions, wind power generation and demand variations) and related interval values. The paper presents a combined AA-P-OPF method, which can provide important information to transmission system operators for evaluating the trade-off between security and costs at a planning stage, as well as for selecting optimal controls at operational stage. The AA-P-OPF methodology is illustrated for a day-ahead planning, using a case study of a real transmission network and a medium size test distribution network.

A review on the regional collaboration of power utilisation in South Asia

Economic prosperity and development in South Asia are significantly restricted by the insufficient power supply. Inadequate indigenous energy resource of each South Asian nation is not enough to generate power for meeting the growing demand. Moreover, the availability of limited indigenous energy resources differs from one country to another in South Asia. In addition to availability variation of indigenous energy sources, the climate conditions also vary resulting in variable power demand and power generation capacity. This paper reviews the current power generation capacity of all eight nations of South Asia. It highlights the bilateral and multilateral collaboration opportunities in power generation and the need for cross boarder power trade among South Asian nations. The review emphasises that the bilateral and multi-lateral cooperation will allow overcoming regional power shortage and augmenting collective energy security, economic and political integration.

Control Method of Self-Frequency Recovery and Active Power Sharing for an Isolated Microgrid Based on VSGs

The concept of virtual synchronous generator (VSG) is emerging as an attractive solution for renewable energies to enhance the microgrid inertia and damping property by emulating the essential behaviors of conventional synchronous generators. However, in the isolated microgrid based on VSGs, there will be a frequency deviation when the power demand varies because of the droop characteristic of the VSG in active power–frequency regulation. In this paper, a control method for VSGs to implement self-frequency recovery and active power sharing in isolated microgrid is proposed. With this method, the VSGs regulate their power baseline values over multiple iterations through local-feedback of the frequency deviation until the microgrid frequency restores to normal value, and simultaneously share the power demand variation according to the ratio of their power baseline values. Compared to the previous studies, the proposed method can eliminate the output power and frequency oscillations in secondary control, and additionally, reduce the dependence of the microgrid on communication system, which is beneficial to enhance the robustness of the microgrid operation. Simulation results proved the proposed method to be effective.

Liquids‐to‐Power Using Low‐Temperature Solid Oxide Fuel Cells

AbstractSolid oxide fuel cells (SOFCs) operating at high temperatures (>800 °C) have been overlooked in the past for automotive applications, primarily due to their longer start‐up times and inability to load follow (quickly respond to varying power demands). However, lower operating temperature SOFCs (LT‐SOFCs) enable more rapid start‐up and several other benefits. The rapid deployment of cost‐effective LT‐SOFC technology for automotive applications relies on the utilization of conventional ‘‘drop‐in’’ fuels (e. g., gasoline, diesel, ethanol etc.), that adapts the existing fuel distribution infrastructure. In particular, the liquid‐fed LT‐SOFCs have the potential to deliver higher “well‐to‐wheels” fuel conversion efficiencies than H2‐fed fuel cells as well as take advantage of the higher fuel energy density. Therefore, this review focuses on the benefits/challenges of low‐temperature SOFCs (350–650 °C) running on liquid fuels. The critical role of anodes and requirements/status of advanced anode materials for the direct utilization of liquid fuels in LT‐SOFCs are highlighted.

Co-Optimization of Energy and Reserve Capacity Considering Renewable Energy Unit with Uncertainty

This paper proposes a system model for optimal dispatch of the energy and reserve capacity considering uncertain load demand and unsteady power generation. This implicates uncertainty in managing the power demand along with the consideration of utility, user and environmental objectives. The model takes into consideration a day-ahead electricity market that involves the varying power demand bids and generates a required amount of energy in addition with reserve capacity. The lost opportunity cost is also considered and incorporated within the context of expected load not served. Then, the effects of combined and separate dispatching the energy and reserve are investigated. The nonlinear cost curves have been addressed by optimizing the objective function using robust optimization technique. Finally, various cases in accordance with underlying parameters have been considered in order to conduct and evaluate numerical results. Simulation results show the effectiveness of proposed scheduling model in terms of reduced cost and system stability.

A MILP methodology to optimize sizing of PV - Wind renewable energy systems

This paper proposes a methodology that is based on mixed-integer linear programming (MILP) to calculate the optimal sizing of a hybrid wind-photovoltaic power plant in an industrial area. The proposed methodology considers the: i) load requirements; ii) physical and geometric constraints for the renewable plants installation; iii) operating and maintenance costs of both wind and PV power plants; and the iv) electric energy absorbed by the public network.The power demand variation associated with the production cycles is considered by using a stochastic simulation tool. To consider both the load and seasonality variability, and to adapt the methodology to the actual operational use of the power plant, the optimization is performed separately for each month of the year. Then, an integrated economic analysis is discussed. The methodology is adopted to analyze an industrial plant in the Rome area used as a train depot and for maintenance purposes. The results, which combine the needs of the plant activity with the availability of renewable energy, enabled the determination of optimal solutions and the relevant savings achievable.

Economic analysis of heat and electricity production in combined heat and power plant equipped with steam and water boilers and natural gas engines

This paper deals with the cost and energy efficiency for heat and electricity production. The operation of the medium size Combined Heat and Power plant, located in Poland, with diverse production units: steam boilers, water boilers and gas engines is studied. The analysis is performed for spring-autumn season, characterized by relatively low and variable heat power demand. Due to such mutable conditions production units are forced to operate out of the nominal efficiencies. The analysis is performed for heating and electric energy production on: steam boilers and turbo-generator, water boilers, steam boiler and gas engines; and water boiler and gas engines. To reflect the actual situation, a mathematical model for the complete installation is developed. The model calculates energy balance for a given heating power demand profile. The model is based on the actual parameters, including all technical aspects and equipment’s limitations. The developed tool enables to select the mode of operation for each production unit separately or in combined mode. As a result, the energy balance in one hour operation period is calculated. Such approach allows to compare the energy production efficiency and cost in each system at every hour.Due to the fact, that nowadays heat and electricity production business, is strongly affected by legal regulations, the analysis of gas engines profitability with and without subsidy for natural gas based co-generation is performed.

Dynamic Optimization of a Subcritical Steam Power Plant Under Time-Varying Power Load

The increasing variability in power plant load in response to a wildly uncertain electricity market and the need to to mitigate CO2 emissions, lead power plant operators to explore advanced options for efficiency optimization. Model-based, system-scale dynamic simulation and optimization are useful tools in this effort and are the subjects of the work presented here. In prior work, a dynamic model validated against steady-state data from a 605 MW subcritical power plant was presented. This power plant model was used as a test-bed for dynamic simulations, in which the coal load was regulated to satisfy a varying power demand. Plant-level control regulated the plant load to match an anticipated trajectory of the power demand. The efficiency of the power plant’s operation at varying loads was optimized through a supervisory control architecture that performs set point optimization on the regulatory controllers. Dynamic optimization problems were formulated to search for optimal time-varying input trajectories that satisfy operability and safety constraints during the transition between plant states. An improvement in time-averaged efficiency of up to 1.8% points was shown to be feasible with corresponding savings in coal consumption of 184.8 tons/day and a carbon footprint decrease of 0.035 kg/kWh.

Analysis of energy efficiency and thermal comfort for an office building complex located in Poland – a case study

This article presents an analysis of energy demand for an office buildings complex located in Gdansk, Poland. The analyses were performed with advanced computer simulations using the Energy Plus software. Simulations were validated by comparing their results with the actual energy consumption data for one of the buildings belonging to the complex and information gathered from complex owner and technical office. The paper includes the effectiveness analysis of using various sources and distribution systems for heating/cooling, use of the phase–change material as internal ceiling layer, reflective glass application and use solar energy produced by photovoltaic glass and PV panels located on the roofs. For each of the variants, thermal comfort analysis was carried out. The aim of the analysis is to examine the time variation and peak load reductions of power demand for the considered variants, indicated as possible to be introduced by the complex owner.

Analysis and Control of Optimal Power Distribution for Multi-Objective Wireless Charging Systems

This paper proposes an optimal power distribution method for multi-objective wireless power transfer (WPT) systems, aiming to improve the transmission flexibility and satisfy various power demands. Previous studies have barely explored the impact of wide-range and unpredictable variations of load parameters on the performance of multi-channel power transmissions. In this paper, by taking the impact of equivalent impedance, motion, power demand variation and response speed of charging object into consideration, an optimal power distribution scheme with fast response, high stability and high accuracy is proposed to satisfy various power demands from multiple objectives including portable electronics, moving electric vehicles (EVs), batteries and super capacitors, without using any communication networks. The effectiveness of the proposed control scheme is demonstrated by simulation results based on different charging cases and experimental results based on a 10 W prototype.

A model predictive control strategy of PV-Battery microgrid under variable power generations and load conditions

Renewable energy sources have been increasingly deployed as distributed generators in remote areas. Meanwhile, fluctuating power generation from renewable energy sources, together with variable power demand, poses challenges in stable and reliable power supply. In this paper, a microgrid with solar photovoltaic (PV) and battery energy storage (BES) is studied. A state of charge (SOC)-oriented charging scheme is developed to control the BES to smooth the PV output. Most importantly, a sophisticated control algorithm, consisting of a model predictive voltage control (MPVC) and a model predictive power control (MPPC), is proposed for the interlinking converter. It enables stable voltage in islanded mode. Also, in grid-connected mode, flexible reactive power can be injected into the main grid for grid support according to the voltage variation level. Finally, by considering the intermittent nature of the PV and the load profile, an energy management system (EMS) is designed to ensure power balance within the system. Case studies are provided to demonstrate the effectiveness of the proposed control strategy.

Battery-like Response in Solid Oxide Fuel Cell Device

The intermittency of renewable energy sources and the varying power demand throughout the day pose challenges for electrical grid stability as more and more renewable energy technologies are deployed. Existing solid oxide fuel cell technologies are slow in response to changes in current and voltage, and therefore cannot smooth out the difference between the power supply from renewables and the power demand. In this presentation, we will report a battery-like response as a new functionality incorporated into the fuel cell devices. It is realized by having in situ charge storage (e.g., metal hydrides) proximal to or integrated within the anode of the fuel cell. Electrochemical studies show a capacity density of 977 mAh g-1 in a 90-hour stable discharge. In addition, a fuel cell-battery hybrid electrochemical device was designed, and the voltage was stable at 0.86 V in the fuel cell mode and 0.73 V in the battery mode for 100 cycles over a course of 75 hours. Fuel cell with a battery mode represents a promising approach for easing the integration of intermittent renewables and enabling grid stability.

Sustainable Development of Regional Power Systems and the Consumption of Electric Energy

Nowadays, one of the most imminent problems facing power systems in post-industrial countries is the sustainable development of power systems under conditions of increasing power consumption irregularity due to the reduction of the industry’s share in consumers’ demand for electric power. In today’s Russia, this issue is becoming very acute due to the significant share of electric power and heat co-generation that is demonstrating low manoeuvrability and poor adaptation to operations in the daily variation of electric power demand. This paper considers the problem of improving the power system steady-state through the optimization of the production structure of thermal power plants. We propose a combinatorial algorithm that improves the planning of the structural and technological modernization of the power equipment configuration, with a glance at the forecast of the increasing irregularity of power consumption.

Energy Savings in Coastal Fisheries: Use of a Serial Battery Hybrid Power System

Fuel effectivization and emission reductions have been on the forefront of research for marine power system solutions. The reduction in fuel use was originally motivated by savings on the principal operating cost of vessels. The need to reduce emissions, both CO2 for the global environment and nitrous oxides (NOx) and soot for the local environment, is on the agenda for the maritime industry. The introduction of shore power supply for ships in port and the NOx emission tax and reduction support scheme are examples of this in Norway. Another initiative that is intended to reduce emissions is the development of battery-based propulsion systems with electric power distribution. Unfortunately, the use of an electric power distribution system, with power conversions from a diesel generator and subsequent storage and retrieval from battery banks, will inevitably result in power losses. However, the actual operation of normal diesel-powered ships is seldom optimal, and only larger ocean-crossing vessels may hope to operate their prime mover and support power systems in an optimal fashion for prolonged periods of time. This article presents the application of a serial-diesel-battery hybrid power system installed onboard a Norwegian costal fisheries vessel, where the variation in power demand allows the batteries to replace the traditional diesel power used in fisheries. A power and propulsion system design is presented as well as energy and time use data for the vessel's operation during one calendar year. The simple design of vessels, such as of the example vessels in this article, allows battery hybrid power systems to deliver energy savings due to the low-power demand of the diesel engine operation at idle power used during the operation of the fishing gear. The prime mover is used to supply limited propulsion power, which is mainly hydraulic and electric. The power system for the example vessel is designed as a serial hybrid where the diesel engine is exclusively used to supply electric power to the batteries, which has the added benefit of shielding the diesel engine from load variations on the propeller. The hybrid battery power system is expected to cut operating costs by minimizing fuel consumption during fishing, reducing the use of engine lubricants, diminishing maintenance costs by requiring fewer engine-running hours, and increasing the lifetime of the onboard diesel engine in calendar days due to decreased running time.

Control System Design of Power Tracking for PEM Fuel Cell Automotive Application

AbstractOwing to load variations, the power tracking of fuel cells in automotive applications is emphasized. We address the control strategies to meet varying power demands while optimizing system efficiency. Based on the control‐oriented proton exchange membrane fuel cell model, the relative gain array is analyzed to determine variable parings in control design. As the dynamics of two control variables are different, a cascade control architecture is adopted. A feedforward‐feedback composite control is used to achieve fast response and eliminate the stable error. To avoid oxygen starvation at power transients, an oxygen excess ratio constrained strategy is proposed to limit the load current if there is lack of reaction oxygen. A mixed‐sensitivity synthesis is applied to suppress parameter perturbation and improve system robustness. To further suppress the fluctuations of the oxygen excess ratio and achieve the maximum system efficiency, an oxygen excess ratio invariant strategy is developed, which coordinates load current with airflow. A comparative study is conducted, with two scenarios of power tracking and fuel cell degradation. The results show that the mixed‐sensitivity strategy has a fast response in power tracking and effective perturbation suppression performance, while the oxygen excess ratio invariant strategy maximizes the system efficiency.

Stochastic modelling of aggregated thermal loads for impact analysis of demand side frequency regulation in the case of Sardinia in 2020

This paper proposes a model of the thermal dynamics and of the end-use of domestic refrigerators (fridges) and water heaters (boilers) in the foretasted scenario of the Sardinian electric network in 2020. This model is employed to evaluate the potential variations of power demand of the aggregates of fridges and boilers during one year in the considered scenario. The resulting quantities can be considered as a form of power reserves to be used for contributing to the frequency regulation through a proper demand side response strategy. The particular case of the system for frequency control proposed by the European Network of Transmission System Operators for Electricity (ENTSO-E) is then analysed by simulations in order to show advantages and drawbacks.

Dynamic power supply by hydrogen bound to a liquid organic hydrogen carrier

Liquid Organic Hydrogen Carriers (LOHCs) are able to store hydrogen in a dense and safe form at ambient conditions. While storage of electrical energy in these carrier systems is one possible and attractive application, the dynamics of the load profile has been believed to represent a major challenge for this storage technology. Conversely, we report here that storage systems based on the LOHC technology are indeed able to deal with significant variations in power demand. This is due to the significant free volume in the LOHC release unit offering the opportunity to handle dynamic behavior by pressure changes. While pressure changes allow quick adaption of the power release on demand, changes in the reactor temperature lead to slow modification of the power output, as demonstrated in this work for hydrogen release from perhydro-dibenzyltoluene (H18-DBT).